The Smithsonian announced Wednesday that veterinarians performing an ultrasound on Mei Ziang (may-SHONG) detected what they believe is a developing giant panda fetus. The zoo says that based on the size of the fetus, veterinarians think Mei Xiang could give birth early next week or possibly in early September.

But the zoo says there is a substantial possibility that Mei Xiang could resorb or miscarry a fetus. Scientists do not fully understand why some mammals resorb fetuses.

Though Mei Xiang has given birth to cubs in the past, the zoo says historically she has refused to participate in ultrasounds at this stage.

The Zoo’s panda team is monitoring Mei Xiang through the Zoo’s panda cams. She is continuing to spend more time in her den, sleeping more, body licking and cradling objects – all behaviors they say are consistent with a pregnancy or “pseudo-pregnancy,” including being very sensitive to noise.

“Today, we are cautiously optimistic,” said Dennis Kelly, director of the Smithsonian’s National Zoo. “We want a healthy cub for all the right conservation reasons. I am excited, but I have to say that we were prepared for a cub even before this morning’s ultrasound. Our expert team of keepers, scientists and veterinarians are going to do exactly what they are trained to do and I’ll just ask everyone to remain positive with us.”

Mei Xiang was artificially inseminated in the spring. This would be her third cub if she is pregnant. One of her cubs lives in China. The other, Bao Bao, is a popular resident at the National Zoo.

Mei Xiang’s first surviving cub, Tai Shan, was born in 2005. Bao Bao, her second, turns 2 on Sunday.

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